1. Field of the Invention
This invention relates to a method and apparatus for reduction of nitrogen oxide emissions by industrial heating furnaces, such as utility boilers, fired by carbonaceous fuels, in particular, fuels with fixed nitrogen, such as coal.
2. Description of Prior Art
During the combustion of fuels having fixed nitrogen, such as coal, oxygen from the air combines with the nitrogen to produce nitrogen oxide (NO). At sufficiently high temperatures, oxygen also reacts directly with atmospheric nitrogen to form nitrogen oxide. A small fraction of the nitrogen oxide formed in the flame is oxidized to nitrogen dioxide (NO.sub.2) downstream of the flame. The total emission of nitrogen oxides, NO+NO.sub.2, is denoted by NO.sub.x. The generation and emission of nitrogen oxides are undesirable because they are toxic. In addition, nitrogen oxides, along with oxides of sulfur (SO.sub.2, SO.sub.3) contribute to acid rain precipitation, and in the presence of sunlight, react with hydrocarbons to produce photochemical smog and ozone.
The 1990 Clean Air Act amendments require industrial furnace operators to reduce nitrogen oxide emissions from fossil fuel-fired furnaces under Title IV of the Act. In addition, measures for ozone attainment under Title I of the 1990 Clean Air Act amendments are also being established. Because nitrogen oxides contribute to tropospheric ozone formation, limitations on NO.sub.x emissions are more stringent during the summer "ozone season." Consequently, many industrial furnace operators that have installed low NO.sub.x burner/overfire air systems or post-combustion control systems will require additional NO.sub.x controls, particularly during the summer months. Thus, there is a need for methods and apparatuses which reduce the nitrogen oxide emissions from such industrial furnace facilities.
Commercially available techniques for reducing nitrogen oxide emissions in furnace flue gases include low-NO.sub.x burners, overfire air, selective non-catalytic NO.sub.x reduction (SNCR), selective catalytic reduction (SNCR), and reburning.
Reburning, that is, in-furnace nitrogen oxide reduction or fuel staging, has been described in several patents and publications. See for example, "Enhancing the Use of Coals by Gas Reburning Sorbent Injection," presented at the Energy and Environmental Research Corporation (EER), First Industry Panel Meeting, Pittsburgh, Pa., Mar. 15, 1988; "GR-SI Process Design Studies for Hennepin Unit No. 1-Project Review," Energy and Environmental Research Corporation (EER), presented at the Project Review Meeting on Jun. 15-16, 1988; "Reduction of Sulfur Trioxide and Nitrogen Oxides By Secondary Fuel Injection," Wendt, et al., published at the Symposium of the Combustion Institute, 1972; "Mitsubishi" MACT In-Furnace NO.sub.x Removal Process For Steam Generator," Sakai et al., published at the U.S.-Japan NO.sub.x Information Exchange, Tokyo, Japan, May 25-30, 1981.
Reburning is a technique whereby a fraction of the total thermal input to the furnace is injected above the primary combustion zone to create a fuel rich zone. Hydrocarbon fuels such as coal, oil, or gas are more effective NO.sub.x reducers than non-carbon containing fuels such as hydrogen or non-hydrogen containing fuels such as carbon monoxide. Stoichiometry of about 0.90 (10% excess fuel) in the reburn zone is considered optimum for NO.sub.x control. Thus, it is apparent that the amount of reburn fuel required for effective NO.sub.x control is directly related to the stoichiometry of the primary combustion zone and, in particular, the amount of excess air therein. Under typical furnace conditions, a reburn fuel input of over 10% of the total fuel input to the furnace is usually sufficient to form a fuel-rich reburn zone. The reburn fuel is injected at high temperatures in order to promote reactions under the overall fuel-rich stoichiometry. Typical flue gas temperatures at the injection point are above 2600.degree. F. Overfire air is introduced into the flue gases downstream of the fuel-rich reburn zone in order to complete combustion of any unburned hydrocarbons and carbon monoxide (CO) remaining in the flue gases leaving the fuel-rich reburn zone. In addition, it is also known that rapid and complete dispersion of the reburn fuel in the flue gases is beneficial. Thus, the injection of reburn fuel is frequently accompanied by the injection of a carrier fluid, such as recirculated flue gases, for the purpose of promoting mixing. To the extent that the recirculated flue gas contains oxygen, the amount of reburn fuel must be increased. Furthermore, due to the requirement of at least three combustion zones within the furnace, a primary combustion zone, a fuel-rich reburn zone downstream of the primary combustion zone, and a fuel-lean completion zone downstream of the fuel-rich reburn zone, implementation of conventional reburn technology requires a relatively tall furnace. Unfortunately, many furnaces in the United States do not have the internal volume required for retrofitting this technology.
Full scale demonstrations of conventional natural gas reburning with flue gas recirculation for mixing and overfire air for completion of combustion of unburned hydrocarbons and carbon monoxide have shown up to 65% NO reduction under the high temperature fuel-rich conditions in several cyclone, wall, and tangentially fired boilers. However, conventional natural gas reburn is expensive due to the capital and operating expenses required for recirculating flue gases and providing overfire air (also known as burn-out air). In addition, the requirement of a fuel-rich zone suggests the use of greater than 10% natural gas, making conventional gas reburn uneconomical for most coal-fired furnaces.
Because it is cheaper than natural gas, coal has also been used as a reburn fuel. However, coal reburn requires a finer grind of coal than the typical utility grind in order to improve coal devolatilization and char burnout in the upper furnace which is diminished by the lack of O.sub.2 inherent in the fuel-rich zone requirement. Because coal has inherent bound nitrogen which can be oxidized to NO, the use of coal as a reburn fuel is limited to initial NO concentrations greater than 200 ppm. This effectively precludes the use of coal reburn in many furnaces equipped with low-NO.sub.x burners.
Selective non-catalytic reduction (SNCR) processes for NO.sub.x reduction are based on the injection of chemical reagents into the combustion flue gases. In these processes, NO.sub.x, substantially all of which is NO, is reduced to nitrogen, N.sub.2, by injection into the flue gases of a nitrogenous compound such as ammonia (NH.sub.3), urea (NH.sub.2).sub.2 CO, or cyanuric acid (HNCO).sub.3. All of these compounds either directly or indirectly form NH.sub.i radicals which subsequently react with NO in the flue gases to produce N.sub.2. These processes are called "selective" because the chemical reagents selectively react with NO. Thus, only small amounts of the ammonia, urea, or cyanuric acid are required.
A further process for NO.sub.x reduction in combustion flue gases involves the injection of a reburn fuel, such as natural gas, into the flue gases downstream of the primary combustion zone so as to maintain an overall fuel lean stoichiometry in the upper furnace. The chemical kinetic mechanisms of this fuel lean gas reburn process and the selective non-catalytic reduction process have many of the same selective reactions. The injection of natural gas in hot, low oxygen furnace gases results in the formation of hydrocarbon radicals (CH.sub.i), and the injection of urea results in the formation of amine radicals (NH.sub.i). Both of these radicals reduce NO to molecular nitrogen through a series of very similar selective reactions.
The selective non-catalytic reduction process reactions are highly efficient in reducing NO.sub.x in a narrow temperature window of about 1700.degree. F. to about 1900.degree. F. At higher temperatures, the process performance drops off due to oxidation of the amine additive to NO. At lower temperatures, the kinetics are too slow and result in high reagent leakage. At very high initial NO.sub.x levels, the selective non-catalytic reduction process may be effective at temperatures somewhat above 1900.degree. F., but usually below about 2100.degree. F.
Due to the high efficiency of the selective reactions between the NH.sub.i radicals and NO, very small quantities of the reagents are needed. The key to acceptable selective non-catalytic reduction process performance is good mixing and reagent dispersion in the flue gas, and injection of the reagents into the proper temperature zones. Typically, use of a normalized stoichiometric ratio (moles of N-atom injected/mole of NO in the flue gas) of slightly greater than 1.0 results in significant NO reduction under optimum conditions of flue gas temperature, O.sub.2 concentration, and the mixing of the reducing nitrogenous reagent with the flue gases. Because of the small quantities of these reagents injected, the use of a carrier agent, such as liquid water, air, steam, or recirculated flue gas, is required to achieve the desired jet penetration and mixing. The presence of residual quantities of oxygen, which are normally present in the flue gas, is required for initiating the formation of amine-type radicals from the nitrogenous reducing agents. Because the selective reduction reactions of NO compete with the oxidation of ammonia or of other nitrogenous agents, the flue gas temperature at the point of chemical reaction should not exceed about 1900.degree. F. Conversely, in the absence of added promoters or modifiers, the lower value of the temperature window is limited to about 1600.degree. F. At lower temperatures, the rate of reaction of the reducing agents with NO is too slow, thereby resulting in inadequate NO reduction and in "ammonia slip," which, in turn, can result in the deposition of corrosive ammonium bisulfate and ammonium sulfate on the air heater surfaces of boilers. For coal fired boilers, "smelly fly ash" due to absorption of ammonia on the fly ash is another problem which may be caused by the ammonia slip.
The narrow process temperature window is a drawback to the implementation of the selective non-catalytic reduction process. Due to the difficulty in maintaining optimum injection temperature conditions, it may result in lower than the maximum possible NO reductions in a number of practical applications. The shifting nature of the flue gas temperature profile is especially a problem in boilers operating at varying load levels due to electric power dispatch requirements. Two approaches, used alone or in combination, are used by vendors of the selective non-catalytic reduction process to mitigate the problems caused by the shifting location of the temperature window and boilers. One approach is to design the selective non-catalytic reduction system for multiple-stage injection of a reducing agent, shifting the injection location upstream as the flue gas temperature decreases, due to lowering of the boiler load. The other approach is to co-inject free radical precursors or promoters with the ammonia or other nitrogenous reducing agents. These promoters can modulate the temperature window to lower temperatures down to about 1300.degree. F. Thus, while the width of the optimum temperature window remains about 200.degree. F., in practice, it can be effectively "broadened" to about 400.degree. F.-500.degree. F. through the use of promoters such as hydrogen, hydrocarbons, or carbon monoxide. However, the practical lower temperature limit of selective non-catalytic reduction operation with CO or hydrocarbon promoters is about 1500.degree. F.-1600.degree. F., below which the rate of oxidation of CO is too slow. Selective non-catalytic reduction operation above 1500.degree. F.-1600.degree. F. is also desirable for limiting the emission of nitrous oxides (N.sub.2 O) as a by-product. While not a problem with ammonia as the selective reducing agent, nitrous oxide is a greenhouse gas and, thus, its emission from some selective non-catalytic reduction process is of concern.
As previously stated, due to the small quantities of reducing nitrogenous reagents injected into the flue gases, the use of a carrier agent is required to achieve the desired jet penetration and mixing. Using natural gas as a carrier for the amine reagent widens the acceptable reaction temperature window in comparison to the selective non-catalytic reduction process, allows amine injection at higher temperatures without amine oxidation to NO, and improves the kinetic rates of the critical chemical reduction mechanisms. The natural gas creates a locally reducing environment for the amine chemistry that raises the acceptable temperature window and prevents the amine-oxidation reactions. Finally, the natural gas lowers the average oxygen concentration which generally improves the final amine reduction efficiency. Completion of the reactions at higher temperatures also decreases the chances of "ammonia slip", a by-product of both selective non-catalytic reduction and selective catalytic reduction processes.
U.S. Pat. No. 5,443,805 teaches injection of an additive such as ammonia with a small amount of hydrocarbon, preferably methane or natural gas, into flue gases at a temperature in the range of about 1750.degree. F.-2100.degree. F., and preferably 1800.degree. F.-1950.degree. F., for reducing pollutants such as NO.sub.x therein. As claimed herein, hydrocarbon is injected for the purpose of enhancing the NO.sub.x reduction efficiency of the nitrogenous NO.sub.x -reducing additive in the temperature range of about 1300.degree. F. to about 2100.degree. F. Injection of the additive in accordance with the '805 patent is achieved by atomization of a liquid-form additive or additive solution with a small amount of the gaseous hydrocarbon. The concentration ratio (or molar ratio on a volume to volume basis (ppm/ppm)) of the hydrocarbon to the additive is between 0.2 and 0.01, and preferably about 0.1-0.03. The amount of additive is selected such that the molar ratio of additive to NO.sub.x in the flue gases is about 2.0 or less, preferably, about 1.0-1.5. Injection is carried out in a single stage such that the additive and hydrocarbon are present in the same physical region of the flue gases, a fuel-lean region, exposed simultaneously to substantially the same temperature regime. Typically, the hydrocarbon comprises up to about 0.5%-15%, and most preferably about 5%, by weight of the injected gas/additive mixture.
The method is indicated to be effective in reducing not only NO.sub.x, but other species containing bound nitrogen, that is, the total bound nitrogen (TBN) which are further potential sources for the formation of NO.sub.x by oxidation. The high efficiency of TBN reduction taught by the '805 patent is attributed to atomization, mixing and distribution of proper molar ratios of additive to NO.sub.x and hydrocarbon to additive that enhance the kinetics of the NO.sub.x -reducing reactions in effluent streams with rapidly changing temperature. The benefit of the approach taught by the '805 patent is that improved reduction of TBN occurs with the addition of small amounts of hydrocarbon fuel with nitrogenous additives at lower temperatures than are possible through the addition of nitrogenous additives alone. The addition of hydrocarbon fuel with nitrogenous additives also has the adverse effect of increasing the emissions (reducing the reduction) of TBN species at higher temperatures (above about 1800.degree. F.).
U.S. Pat. No. 5,756,059 teaches a method and system for preventing the release of nitrogen oxides with combustion flue gases emitted to the atmosphere by stationary combustion systems using conventional and advanced reburning processes utilizing injection of a reducing agent into the reburning zone and the use of promoters which considerably enhance NO.sub.x control. The promoters or metal-containing compounds can be added to the reducing agent and injected into the furnace through overfire air. The introduction of overfire air above the reburn zone is required in order to ensure complete combustion of combustibles remaining in the flue gases downstream of the reburn zone.
U.S. Pat. No. 4,325,924 teaches a method of reducing NO.sub.x in fuel rich combustion effluents in which urea is introduced into the fuel rich combustion effluents at temperatures in excess of about 1900.degree. F. in the presence of excess fuel wherein the urea is introduced either as a solid or solution in amounts sufficient to reduce the NO.sub.x concentration. The equivalence of fuel to oxygen in the flue gases is greater than 1 and the amount of urea injected into the flue gases is in the range of about 0.5-10 moles of urea per mole of nitric oxide in the flue gases. This process requires additional downstream air to prevent excessive emissions of CO and other partially burned products.
U.S. Pat. No. 5,139,755 teaches a method and system for reducing oxides of nitrogen from combustion flue gases by creating an overall fuel-rich zone above the primary combustion zone and two burnout zones disposed above the overall fuel-rich zone. In the first burnout zone, CO is reduced to below 0.5% and, in the second burnout zone, the remaining combustibles are oxidized. Nitrogenous additives may be added through either overfire air duct in the first or second burnout zones.
U.S. Pat. No. 4,861,567 teaches a method of reducing NO.sub.x and SO.sub.x emissions from combustion systems by adding cyanuric acid to a fuel-rich, oxygen-free zone. The resulting decomposed cyanuric acid and fuel-rich zone reaction products are mixed with the effluent stream of a combustion system containing NO.sub.x. At this point, the oxygen level must be maintained sufficiently high to assure complete burnout of combustibles, which may require the injection of air.